1. Field of the Invention
This invention relates to metal conductors having improved retention of solderability on storage.
2. Introduction to the Invention
When metal conductors are stored, especially in humid environments, their solderability decreases. This causes the most serious problems in the electrical and electronics industries, and much effort has been devoted to finding solutions to those problems. Reference may be made for example to the Naval Weapons Center Technical Publication 6707 (NWC TP 6707), which is a collection of the papers presented at the Tenth Annual Soldering/Manufacturing Seminar in February, 1986, and the disclosure of which is incorporated herein by reference. A quantitative measure of the rate at which the solderability of a metal conductor decreases can be obtained through the test defined in Military Standard 202F, Method 208D (Apr. 1, 1980) and its successors, referred to herein as MS 202F-208, the disclosures of which are incorporated herein by reference. In this test, the conductor is subjected to steam-aging for a specified time (thus simulating aging for a much longer time under normal storage conditions). The aged specimen is then dipped successively in a flux and in a solder under specified conditions, and the percentage of the dipped conductor which is covered by a continuous new solder coating is measured. According to Method 208D, the conductor, in order to pass the test, must be at least 95% covered after steam aging for 1 hour. However, it has been found that in many cases conductors which pass the test under these conditions are nonetheless unsatisfactory in solderability after storage under normal conditions. Consequently it has recently been proposed that that the duration of the steam-aging step should be increased to 8 hours, or even to 16 or 24 hours, in order to ensure satisfactory solderability. It has been suggested that tin-coated copper stranded wires will have improved retention of solderability if the tin-coating is at least 40 microinches thick, i.e. well above the usual thickness of 20-25 microinches. But it is difficult to ensure that there is a uniform thickness of tin on each strand of the wire, and while the thicker coating may help, it does not reliably enable the wire to pass MS 202F-208 at 8 hours of steam-aging.